The curtain rose on Michael Peterson's murder defense Monday with a neuropathologist who testified that Kathleen Peterson died after falling down not once, but twice, in the steep stairway where she was found dead.

"They may be dizzy or slip or do something and essentially do it again," testified Dr. Jan Leestma, saying the double-fall scenario was "not a crazy idea." 

Leestma's testimony Monday — the expert spent most of the day under direct examination by Peterson's lawyer — focused on the defense's theory that the lacerations on Kathleen Peterson's head, no matter how numerous, could still have been caused by an accidental fall. 

A diagram of the lacerations on Peterson's skull

Leetsma asserted that the seven lacerations prosecutors claim were found on the back of her head are a misreading of the wounds. He told the jury that a fall on a flat surface could produce a complex split or tear in the scalp that may look like several separate wounds but is really the product of a single impact. 

In calling Leestma first, defense lawyers for Michael Peterson, 59, a novelist accused of bludgeoning his financial analyst wife to death in December 2001, appear to be striving to explain away the prosecution's most powerful testimony.

Last week Dr. Deborah Radisch wrapped up the prosecution's case by testifying that seven lacerations on the back of Peterson's head and the amount of blood lost were clear indications of a blunt trauma homicide.

Prosecutors claim that Peterson beat his wife and camouflaged the murder as a fall at the bottom of a steep stairwell in their Durham, N.C., home. They say her death bears striking similarities to the stairwell death of Peterson's neighbor in Germany in 1985.  Peterson was never charged in that case, but he was the last person to see family friend Elizabeth Ratliff alive.

Peterson's lawyer, David Rudolf, holds up a diagram of the victim's skull.

Jurors in his case have been thrust into an imromptu course on brain physiology and pathology over the past week.

On Monday, Leestma made a frontal attack on Radisch's findings, drawing on specially stained slides of braincells, skull diagrams and data from more than 250 beating deaths in North Carolina collected by the medical examiner's office.

"Kathleen Peterson's injuries were the result of a fall, and not the result of a beating," said the expert. "I frankly am not left with any other conclusion that I am happy with."

Leestma told the jury that the "robust, reliable" conclusion he arrived at was supported by the facts that Peterson's injuries were:

• Not consistent with the linear lacerations of a cylindrical weapon (a fireplace poker has been suggested as the murder weapon);
• Devoid of underlying injury (which would rule out a beating altogether); and
• Not consistent with the beating-death data collected by the medical examiner.

"Is it your opinion that there's no scenario that would combine strikes to her head with some object ... that would have caused these injuries?" asked Peterson's lawyer, David Rudolf.

"I wouldn't want to say impossible, but I think it's unlikely," said the witness.

Leestma also disagreed with prosecutor's analysis of the death of Elizabeth Ratliff, the German neighbor of Peterson, noting that vascular anomalies in her brain were consistent with some sort of aneurysm or brain hemorrhage, not an assault.

Prosecutors will have a chance to cross-examine Leestma Tuesday, when Peterson's trial resumes at 9:30 a.m. ET.  The trial is being broadcast live by Court TV.